The Collaborative International Dictionary of English v.0.48:

Immensity \Im*men"si*ty\, n.; pl. Immensities. [L. immensitas:
   cf. F. immensit['e].]
   The state or quality of being immense; inlimited or
   immeasurable extension; infinity; vastness in extent or bulk;
   greatness.
   [1913 Webster]

         Lost in the wilds of vast immensity.     --Blackmore.
   [1913 Webster]

         The immensity of the material system.    --I. Taylor.
   [1913 Webster]

WordNet (r) 3.0 (2006):

immensity
    n 1: unusual largeness in size or extent or number [syn:
         enormousness, grandness, greatness, immenseness,
         immensity, sizeableness, vastness, wideness]